ICP-MS is a technique employed for analyzing inorganic elements, in particular metals, and is widely used in many fields including the semiconductor, geological and environmental industries. ICP-MS offers essentially -simultaneous multi-element analysis for most of the periodic table, produces simple mass spectra, exhibits excellent sensitivity and can determine elemental concentrations at the part-per-trillion (ppt) level.
The ICP-MS employs an inductively coupled argon plasma as an ionization source and a mass spectrometer to separate and measure analyte ions formed in the ICP source. Normally, the sample is taken into solution and pumped into a nebulizer, which generates a sample aerosol. The sample aerosol passes into the ICP, where it is desolvated, atomized and ionized. The resulting sample ions are then transferred from the plasma at atmospheric pressure, to the mass spectrometer that is situated inside a vacuum chamber, via a differentially pumped interface. The ions pass through two orifices in the interface, known as sampling and skimmer cones, and are focused into a quadrupole mass analyzer. The analyzer separates the ions based on their mass/charge ratio prior to measurement by an electron multiplier detection system. Each elemental isotope appears at a different mass with a peak intensity directly proportional to the initial concentration of that isotope in the sample; thus elemental concentrations in the sample can be measured.
While ICP-MS is acknowledged to have higher sensitivity and lower detection limits than conventional elemental analysis techniques such as atomic absorption spectrometry (AAS) and ICP atomic emission spectrometry (ICP-AES, it still suffers from spectroscopic interference. For example, polyatomic ions, such as ArCl.sup.+, ArO.sup.+ and C1O.sup.+, which result from various combinations of atomic species present in the plasma, give rise to spectroscopic interference effects that cannot be sufficiently resolved by the quadrupole mass analyzer. In some cases, problems due to spectroscopic interferences can be overcome by applying mathematical corrections. In many applications, however, a strong need exists to reduce or eliminate spectroscopic interferences. As an example, the ICP-MS is considered to be a useful tool in analyzing and determining trace levels of heavy metal contaminants in drinking water. However, the interference from polyatomic species such as ArO.sup.+, C1O.sup.+ and ArAr.sup.+ on Fe, V and Se respectively, makes it difficult, if not impossible, to produce reliable analytical data at the analyte concentrations typically found in drinking water.
One approach to alleviate the problem of spectroscopic interference is to employ a high-resolution mass spectrometer such as a double focusing magnetic sector analyzer, and equipment of this type is available in the market. However, such equipment is complex by nature, much more costly than quadrupole-based systems, and requires very high operator skill level.
It is also known that the performance of the ICP-MS can be improved by employing a collision cell as an interface for transmitting ions from the plasma source to the quadrupole analyzer.sup.1. With the collision cell technique, a gas such as helium is introduced into a hexapole collision cell situated between the interface region and the mass spectrometer region. Due to collisions with the helium atoms inside the collision cell, polyatomic species undergo higher attenuation than the analyte ions, thereby reducing the population of polyatomic species before the ions enter the analyzer. However, this technique adds complexity to ICP-MS instruments, and also requires substantial, additional expenditure.
The Japanese Patent Laid-Open Publication No. H10-40, 857 describes a technique for improving detection limits in ICP-MS. According to the disclosure, the depth of the skimmer cone orifice is increased so as to cause collisions within the orifice that reduce the number of polyatomic species reaching the mass spectrometer. Although the detection limits of some interfered analyte ions can be improved somewhat by this technique, it is difficult to reproducibly fabricate a skimmer cone with the exact orifice depth required.
A technique for reducing -argon matrix ion (Ar.sup.+) interference in the ICP-MS by modifying a conventional sampling interface has also been described.sup.2. In this case hydrogen or argon gas is introduced via a tube inserted into the intermediate vacuum region behind the skimmer cone. It was demonstrated that argon reduced ion intensity at all masses by collision while hydrogen reduced the level of some ions to a lesser extent than argon. In addition, the introduction of hydrogen gas into the interface region between the sampling cone and the skimmer cone was also investigated, but this resulted in the attenuation of the analyte signal and an increase in the (Ar.sup.+)signal.